In general, a vehicle travels by transmitting a power generated by combusting fuel in an internal combustion engine to the wheels through a transmission and the like. The exhaust gas generated by the combustion contains NOx (nitrogen oxides), PM (particulate matter), and the like, and hence should not be released directly to the atmosphere. In this respect, an after treatment device for exhaust gas is provided in an exhaust passage of an internal combustion engine, and a catalyst device supporting a catalyst is provided in the after treatment device. With this catalyst device, a purification treatment is performed on NOx, PM, and the like contained in the exhaust gas. As a catalyst device for purifying NOx, for example, a NOx storage reduction-type catalyst (LNT: lean NOx trap) or a selective reduction-type NOx catalyst (SCR: selective catalytic reduction) is used.
When a vehicle travels normally, i.e, when the air-fuel ratio of the exhaust gas is in a lean state, the NOx storage reduction-type catalyst device oxidizes NO contained in the exhaust gas to NO2 and stores the NO2. When the amount of NOx stored approaches a storage limit, an air-fuel ratio richness control for placing the air-fuel ratio of the exhaust gas in a rich state is performed to release the amount of NOx stored and also reduce the released NOx.
To place the air-fuel ratio of the exhaust gas in a rich state in this air-fuel ratio richness control, post injection is performed based on in-cylinder fuel injection, or fuel is directly injected into the exhaust gas from a fuel injection device provided in the exhaust passage. In this manner, the amount of HCs in the exhaust gas is increased temporarily, and the HCs are combusted with oxygen in the exhaust gas to place the exhaust gas in a rich state.
In addition, an enrichment interval at which the air-fuel ratio richness control is performed is determined based on a timing at which an estimated value (calculated value) of a NOx storage amount has reached an enrichment start threshold value set in association with a storage limit value, a timing based on an elapsed time specified in an enrichment interval time map (base enrichment interval map) regarding the time elapsed from the last air-fuel ratio richness control to the next air-fuel ratio richness control, or the like.
Meanwhile, a continuation time for which a rich state is continued in a single execution of the air-fuel ratio richness control is determined based on a timing where an estimated value (calculated value) of the NOx reduction amount has reached an enrichment finish threshold value (target reduction amount threshold value), or on a timing based on a time specified in an enrichment continuation time map regarding a time elapsed from the start of the air-fuel ratio richness control, or the like.
However, during warming-up immediately after the ignition of the engine or the like, the combustion in a cylinder in the rich state tends to be unstable, and hence a situation arises in which the air-fuel ratio richness control cannot be performed, although the NOx storage amount becomes not smaller than the threshold value.
For this reason, in a situation where the NOx storage amount is not smaller than the threshold value, a conventional technology employs a control as shown in FIG. 4. Specifically, the air-fuel ratio richness control is prohibited, until the warming-up of the internal combustion engine is finished. The prohibition of the air-fuel ratio richness control is withdrawn after completion of the warming-up. In addition, immediately after the withdrawal of the prohibition, the air-fuel ratio richness control is performed frequently at short intervals to release the excessively stored NOx and purify the NOx by reduction. In this manner, the state where the NOx purification rate is deteriorated is altered, as soon as possible.
However, when the air-fuel ratio richness control is performed at such short intervals in a state where the catalyst temperature is low immediately after the ignition during traveling on an actual road, HCs are excessively supplied to the exhaust passage by the air-fuel ratio richness control performed frequently, which causes a problem of white smoke development. In addition, when the temperature of the catalyst is low, the chemical reaction rate of the catalyst is low. Hence, when the air-fuel ratio richness control is performed with a low catalyst temperature in the same manner as in an ordinary case, there arises such a problem that the redox reaction of the HCs is so insufficient that an HC slip occurs in which the excessive HCs are released to the atmosphere.
In this respect, for example, as described in Japanese patent application Kokai publication No. 2009-270446, an exhaust gas purification method and an exhaust gas purification system have been proposed which achieve an improvement in terms of the release of HCs to the atmosphere as follows. Specifically, a HC-adsorbing member for adsorbing HCs in exhaust gas is provided in an exhaust passage of an internal combustion engine on a downstream side of a NOx storage reduction-type catalyst. At a NOx regeneration control, when an index temperature indicative of the temperature of the HC-adsorbing member is not higher than a first judgment temperature, the air-fuel ratio of the exhaust gas is set at 0.8 to 1.1 in terms of air excess ratio, when the index temperature is between the first judgment temperature and a second judgment temperature, the air-fuel ratio of the exhaust gas is set at 1.0 to 1.1 in terms of air excess ratio, and when the index temperature is at or above the second judgment temperature, the air-fuel ratio of the exhaust gas is set at 0.8 to 1.1 in terms of air excess ratio.